Condensates of ethane-1-hydroxy-1,1-diphosphonic acid are known such as those described in U.S. Pat. Nos. 3,387,024; 3,400,151 and 3,621,081. In the first two noted patents, the condensates are limited to dimers of ethane-1-hydroxy-1,1-diphosphonates joined by C-O-C bonds or P-O-P bonds, as opposed to the oligomeric ester compounds of the present invention which contain C-O-P bonds. While the latter mentioned patent U.S. Pat. No. 3,621,081 to Prentice does disclose oligomeric ester compounds of phosphorous acid which contain C-O-P bonds, the patent specifically limits the compounds to those containing a methyl end group. Not only were the preparatory methods of Prentice found to be unsuitable for preparation of the C.sub.6 -C.sub.13 compounds of the present invention, but the higher homologs were unexpectedly found to possess "reverse" emulsion breaking properties not found in the Prentice oligomer. Accordingly, the present invention also relates to the use of these compounds as "reverse" emulsion breakers and to a process for their preparation.